1. Field of the Invention
The invention relates to a method of producing difatty acid diamides of symmetrical composition and/or the corresponding pure fatty acids from fatty acids or fatty acid esters.
2. Technology Review
Difatty acid diamides are compounds with a wide range of technical applications that are used, among other things, as auxiliary materials in plastics processing, as lubricants, or as fluxing agents for solders. Such difatty acid diamides are chiefly made by reacting activated fatty acid mixtures of commercial oils with diamines. The diamides resulting from these kinds of reaction are not uniform compounds in chemical terms. They are rather mixtures of difatty acid diamides as they form according to the various possibilities of combination, depending on the fatty acid composition of the raw materials used. When starting from natural fats and oils, which contain three different kinds of fatty acids, mixtures of six different difatty acid diamides thus result according to the rules of combination, some of which are symmetrical, i.e. are composed of two like fatty acid residues, and others are unsymmetrical. Such difatty acid diamide mixtures of heterogeneous composition can therefore only be used in cases where heterogeneity is not troublesome or is even advantageous. In contrast, pure diolecic acid diamides for example can be used to advantage as lubricating agents for plastics as used, for example, for the sheathing of cables.
Separating fatty acid mixtures usually requires considerable efforts. This is due, among other things, to the structural similarity of the fatty acid molecules to be separated or the reactivity of the frequently preset double bonds. Separation is the more difficult higher purity when purity is required of the end product. Particular problems arise when separating fatty acids of identical chain length which differ only with respect to the number of double bonds. This applies, for example, to the technically very interesting C.sub.18 carboxylic acids, stearic acid, oleic acid, linoleic acid and linolenic acid. Isolating oleic acid from natural fats is thus very toilsome, because absolute separation from other saturated and unsaturated fatty acids is extremely difficult to achieve. Accordingly, the classical method of producing pure oleic acid by low-temperature crystallization of the fatty acids of olive oil or their methyl esters involves much effort and great losses. However, oleic acid is a very good example by which to demonstrate that a higher content of this constituent, e.g., in cosmetic products like sun lotion, improves the quality of these products considerably.